Engine-internal components are increasingly subject to mechanical and thermal loads due to rising levels of specific engine output. At the same time, their properties should improve further in order to contribute to the efficiency of the powertrain. MAHLE has developed a new oil control ring, on the basis of a cast iron ring, that has extremely low friction properties thanks to the combination of innovative design and PVD coating and that remains wear-resistant even under high loads.
In modern engines with high specific output, it is particularly important that as little oil residue as possible remains on the combustion chamber walls. Therefore, it is a prerequisite that the oil control ring is highly flexible and exhibits good conformability behavior. If, for example, cylinder distortion occurs, the ring will still maintain contact with the running surface and retain its effective scraping capability.
In order to increase the flexibility of the rings and to lower tangential loads, the general trend is toward reducing the cross-sectional area of the rings. The contact land area must be adapted at the same time, however, in order to retain the surface pressure of the land, which is ultimately responsible for scraping off the oil. Starting from the originally common land width of about 0.3 mm, current development has arrived at a nominal value of down to 0.1 mm—about the thickness of a sheet of paper, which presents considerable challenges with respect to wear resistance.
Therefore, MAHLE counts on a combination of the so-called X-taper design and a PVD coating. The X-taper design stands for a shallow rise of the lands, in particular. The land width thus remains low even if there is slight abrasion and the surface pressure intact. Since galvanic plating reaches its limits at this land width, a coating made of MIP 230 chromium nitrite is applied to increase wear resistance. This coating also resists abrasive conditions—such as can occur more severely with thermal spray coatings on the cylinder running surface—, features very high thermal stability, and is not sensitive to reactions with additives. Furthermore, it has a lower friction coefficient in comparison with galvanic coatings, resulting in additional advantages in the mixed friction range.
